Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty

[Jan Stancek <jstancek@redhat.com>]

On Wed, Jul 17, 2019 at 10:19:04AM -0400, Waiman Long wrote:
>> If you add a comment to the code outlining the issue (preferably as a litmus
>> test involving sem->count and some shared data which happens to be
>> vmacache_seqnum in your test)), then:
>>
>> Reviewed-by: Will Deacon <will@kernel.org>
>>
>> Thanks,
>>
>> Will
>
>Agreed. A comment just above smp_acquire__after_ctrl_dep() on why this
>is needed will be great.
>
>Other than that,
>
>Acked-by: Waiman Long <longman@redhat.com>
>

litmus test looks a bit long, would following be acceptable?

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 37524a47f002..d9c96651bfc7 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1032,6 +1032,13 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
  		 */
  		if (adjustment && !(atomic_long_read(&sem->count) &
  		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
+			/*
+			 * down_read() issued ACQUIRE on enter, but we can race
+			 * with writer who did RELEASE only after us.
+			 * ACQUIRE here makes sure reader operations happen only
+			 * after all writer ones.
+			 */
+			smp_acquire__after_ctrl_dep();
  			raw_spin_unlock_irq(&sem->wait_lock);
  			rwsem_set_reader_owned(sem);
  			lockevent_inc(rwsem_rlock_fast);


with litmus test in commit log:
----------------------------------- 8< ------------------------------------
C rwsem

{
	atomic_t rwsem_count = ATOMIC_INIT(1);
	int vmacache_seqnum = 10;
}

P0(int *vmacache_seqnum, atomic_t *rwsem_count)
{
	r0 = READ_ONCE(*vmacache_seqnum);
	WRITE_ONCE(*vmacache_seqnum, r0 + 1);
	/* downgrade_write */
	r1 = atomic_fetch_add_release(-1+256, rwsem_count);
}

P1(int *vmacache_seqnum, atomic_t *rwsem_count, spinlock_t *sem_wait_lock)
{
	/* rwsem_read_trylock */
	r0 = atomic_add_return_acquire(256, rwsem_count);
	/* rwsem_down_read_slowpath */
	spin_lock(sem_wait_lock);
	r0 = atomic_read(rwsem_count);
	if ((r0 & 1) == 0) {
		// BUG: needs barrier
		spin_unlock(sem_wait_lock);
		r1 = READ_ONCE(*vmacache_seqnum);
	}
}
exists (1:r1=10)
----------------------------------- 8< ------------------------------------

Thanks,
Jan